The present invention relates to a magnetic disc coupling mechanism for use in disc drive units and, more particularly, to a magnetic disc coupling mechanism suitable for use in a small-sized disc drive unit for driving a small magnetic disc of 2 inches or so in size which is used, for example, in an electronic still camera, the coupling mechanism being of the type in which the center core of the magnetic disc is held on a spindle hub by magnetic force of a permanent magnet such as to be driven by the spindle hub.
FIGS. 3 and 4 show a known magnetic disc coupling mechanism incorporating a permanent magnet for holding the magnetic recording medium. Referring to these Figures, a disc drive unit has a housing 20 adapted to receive a disc cassette 22 having a hard case. The arrangement is such that the disc cassette 22 is manually forced into the housing 20 and then the housing 20 is depressed towards a spindle hub which will be explained later, so that the magnetic disc 21 in the disc cassette is located and held on the spindle hub and coupled to the same. More specifically, when the disc cassette accommodating the disc 21 is received in the housing 20, a magnetic portion 25a constituting a lower portion of the center core 25 of the magnetic disc 21 opposes a ring-shaped permanent magnet 24 provided on an upper portion of the spindle hub 23 which is drivingly connected to a spindle motor. A roller 27 which is biased downwardly by a spring 26 is disposed on the portion of the housing 20 above the center core 25 of the disc 21.
As the housing 20 is depressed, the center core 25 is pressed by the roller 27 as shown in FIG. 4 so that a hole 25b of the center core 25 is forced to fit on the spindle shaft 23a serving as the output shaft of the spindle motor, with the center core 25 attracted and held magnetically by the permanent magnet 24.
This known arrangement, however, suffers a problem in that the load torque on the spindle motor is increased by an amount corresponding to the load imposed by the roller 27 including the biasing force of the spring 26, because the center core 25 is continuously pressed by the roller 27 under the biasing force of the spring 26 during rotation of the spindle hub 23.
The center core of the magnetic disc is forcibly fitted to the spindle shaft by a forcible coupling means including the roller. This known arrangement, therefore, requires a suitable arrangement for moving this forcible coupling means away from the center core after the coupling operation. An example of such an arrangement is shown in Japanese Utility Model Laid-Open No. 135550/1984. This arrangement employs a complicated mechanism incorporating a rotary cam and, therefore, inevitably increases the size of the disc drive unit and complicates the construction of the same. This makes it difficult to obtain a compact and light-weight disc drive unit suitable for use in a portable electronic still camera. Namely, the use of such a large-sized and heavy-weight disc drive unit undesirably increases the size and weight of the camera.